EIGRP Loop-Free Alternate (LFA) Fast Reroute (FRR)

EIGRP Loop-Free Alternate (LFA) Fast Reroute (FRR) is a feature that allows EIGRP to switch to a backup path in less than 50 ms. Fast reroute means we switch to another next hop, Loop-free alternate is an alternative path in the network that is loop free.

Now you might be thinking that this sounds familiar. After all, EIGRP has feasible successors. Those are loop-free alternate paths that EIGRP has calculated. If the successor fails, EIGRP can use a feasible successor right away.

This is true, but there’s one big “gotcha”. EIGRP feasible successors are not installed in the routing table right away. Only the successor route is installed. When the successor fails, EIGRP installs the feasible successor, and this takes time. Fast reroute installs both the successor route and the feasible successor route in the routing table which makes convergence even faster.

IGPs can calculate LFAs using different methods:

Per-link: all prefixes that are reachable through a certain link share the same information, they all use the same next hop address. When we calculate the LFA per link, we calculate a single backup next hop address for all prefixes that use the link. This means that if the primary link fails, all prefixes are switched to a secondary link. The advantage of per-link LFAs is that it’s simple to calculate, it uses less CPU and memory resources. The disadvantage, however, is that once the primary link fails, all traffic to the prefixes that used the primary link are suddenly switched to a single backup link. It is possible that your backup link gets overburdened with this sudden spike of traffic.

Per-prefix: we calculate an LFA for each prefix, in other words for each possible destination. This requires more resources, but it does offer better load balancing. It’s possible that two prefixes currently use the same path, but once the primary link fails, they use different backup next hops. This spreads your traffic more throughout the network.

When you have multiple possible LFAs, fast reroute has to select one LFA. EIGRP will not just select the “best” (lowest metric) feasible successor but uses some “tie-breakers” to select the best LFA. When you use per-prefix LFA, we have these four attributes:

Interface disjoint: don’t select an LFA that uses the same outgoing interface as the primary path. Imagine we have R1, connected to R2 and R3 on a multi-access segment. R1 also has a connection to R4 using a different interface. R2, R3, and R4 all offer a path to a certain destination. R2 is the successor, R3 and R4 are feasible successors. R3 has a better metric. Which feasible successor would you prefer? R1 can reach R2 and R3 using the same interface, R4 through a different interface. There is a risk that once R2 is unreachable that R3 also becomes unreachable (maybe the switch fails). It might be a better idea to prefer R4 instead…I’m going to show you this in an example, later in this lesson.

Line card disjoint: don’t select an LFA that shares a line card with the primary card. The same logic as the interface-disjoint applies here. There is a risk that your line card fails which means that both the successor and feasible successor are unusable.

Lowest repair path metric: don’t select an LFA with a high metric. This ensures that only the lowest metric LFAs remain.

Shared Risk Link Group (SRLG): don’t select LFAs that use the same SRLG. This one requires some more explanation. Imagine we have R1, connected to R2, R3, and R4. R2 and R3 use the same uplink to reach a certain destination. R4 uses a different link to reach the same destination. R2 is the successor, R3 and R4 are feasible successors, but R3 has a better metric. Which feasible successor would you prefer? R3 has a lower metric, but it uses the same uplink…that’s a risk. By adding R2 and R3 to the same SRLG, you can tell EIGRP to prefer R4 since it uses a different uplink. I will show you an example of SRLG on some routers later in this lesson.

You now know the basics of Fast reroute and Loop free alternate. Let’s look at it in action, shall we…

Configuration

Here is the first topology I use to demonstrate the basics of fast reroute:

These four routers run EIGRP; there’s a loopback on R4 with network 4.4.4.4/32. R1 can go through R2 or R3 to get there. The delay on R1’s GigabitEthernet3 interface has been increased so that R2 is our successor and R3 our feasible successor.

Configurations

Want to take a look for yourself? Here you will find the startup configuration of each device.

The feasible successor is already in the forwarding table, saving valuable time when the successor route fails.

Tie Breakers

Let’s also take a look at some examples to help you understand the tie breakers. In the previous example, we only had one feasible successor, so there’s not much to choose. What if we have two or more feasible successors?

Interface Disjoint

Let’s take a closer look at the interface disjoint tie breaker. I use the following topology to demonstrate this:

Above we have five routers, running EIGRP. Behind R5 we have a loopback interface with network 5.5.5.5/32. R1 can use R2, R3, and R4 to reach it. I used an offset-list on R3 and R4 so that R2 is the successor, R3 and R4 two feasible successors where R3 has a slightly better metric.

Configurations

Want to take a look for yourself? Here you will find the startup configuration of each device.

Forum Replies

Got a couple of questions - firstly after a while you stopped using the acronym LFA and used LSA instead - are they in fact the same thing? (I keep thinking of the completely unrelated Link State Advertisment when I read that!)

“When you have multiple possible LSAs, fast reroute has to select one LSA. EIGRP will not just select the “best” (lowest metric) feasible successor but uses some “tie-breakers” to select the best LSA. When you use per-prefix LSA, we have these four attributes:”

Hi Rene and staff,
just adding another comment
I noted in some blogs that sub-interfaces are considered disjointed interfaces by the FRR process
I don’t know if it is true for all IOS (but it seems true for IOS-XE)
I think that it is interesting to point this out (to be verified in new labs with various flavors of IOS ?)
Regards

As you can see, R3 became the repair path (instead of R4) as i just closed and restarted the lab
I don’t understand why this happened, but now this is correct with the theory: R3 is the repair path because of its better metric, and R1 does not know that R2 and R3 are on the same multi-access segment
But with my platform / IOS, i cant se

As you can see, R3 became the repair path (instead of R4) as i just closed and restarted the lab
I don’t understand why this happened, but now this is correct with the theory:

Sometimes GNS3 can behave in this way. It does occasionally occur where some behaviour is strange, but a reboot or a restart of the lab brings about the expected results. Sometimes it just “gets stuck”. It’s one of the quirks of GNS3 that we have to live

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